Photos of the Great Lakes State

The Science of Fall Color

There’s no doubt that the annual fall show that Michigan puts on is one of the best, but did you ever stop to think about the process that causes deciduous trees to change color? Well, here’s your chance…

The Science of Color in Autumn Leaves from the United States National Arboretum explains that process that starts the cascade of events that result in fall color is a growth process that starts in late summer or early autumn. When the nights get long enough, a layer of cells called the abscission layer forms that begins to block transport of materials from the leaf to the branch.

During the growing season, chlorophyll is replaced constantly in the leaves. Chlorophyll breaks down with exposure to light in the same way that colored paper fades in sunlight. The leaves must manufacture new chlorophyll to replace chlorophyll that is lost in this way. In autumn, when the connection between the leaf and the rest of the plant begins to be blocked off, the production of chlorophyll slows and then stops. In a relatively short time period, the chlorophyll disappears completely.

This is when autumn colors are revealed. Chlorophyll normally masks the yellow pigments known as xanthophylls and the orange pigments called carotenoids — both then become visible when the green chlorophyll is gone. These colors are present in the leaf throughout the growing season. Red and purple pigments come from anthocyanins. In the fall anthocyanins are manufactured from the sugars that are trapped in the leaf. In most plants anthocyanins are typically not present during the growing season.

As autumn progresses, the cells in the abscission layer become more dry and corky. The connections between cells become weakened, and the leaves break off with time. Many trees and shrubs lose their leaves when they are still very colorful. Some plants retain a great deal of their foliage through much of the winter, but the leaves do not retain their color for long. Like chlorophyll, the other pigments eventually break down in light or when they are frozen. The only pigments that remain are tannins, which are brown.

The explain that because the starting time of the whole process is dependent on night length, fall colors appear at more or less the same time every year and are not overly dependent on temperature, rainfall or other factors, other than the fact that weather can shorten or prolong the show by stripping leaves from trees.

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